It is well known that the hydrogen content of the molten aluminum affects the porosity of castings made therefrom. Due to a large difference between the solubility of hydrogen in liquid and in solid aluminum, dissolved hydrogen precipitates from the melt during solidification and becomes trapped in the castings. Accordingly, it is essential to monitor the hydrogen level of the melt before the metal is cast to determine if hydrogen removal is necessary. Techniques used heretofore to measure hydrogen content are generally too slow for a practical commercial application. Attempts have been made to develop solid state electrochemical sensors for the rapid on-site determination of hydrogen content. Such sensors employ a solid electrolyte comprising calcium hydride and a reference electrode comprising a mixture of calcium and calcium hydride. The sensor is immersed in the molten aluminum such that the electrolyte comes into direct contact with the melt and a concentration cell is established between the hydrogen partial pressure in the reference electrode and the hydrogen partial pressure in the melt. Such sensors, however, are short lived due to instability of the calcium hydride electrolyte in contact with the melt.
It is an object of the present invention to provide an improved long-lived, responsive, solid-state hydrogen sensor which includes means for isolating the electrolyte from the aluminum melt, for measuring the hydrogen partial pressure in gas phase equilibrium with the melt, and for catalyzing the hydrogen exchange reaction.